In addition, all modern ethnic groups, other than Africans, carry traces of Neanderthal DNA in their genomes, the study says—which at first puzzled the scientists. Though no fossil evidence has been found for Neanderthals and modern humans coexisting in Africa, Neanderthals, like modern humans, are thought to have arisen on the continent.

"If you told an archaeologist that you'd found evidence of gene exchange between Neanderthals and modern humans and asked them to guess which [living] population it was found in, most would say Europeans, because there's well documented archaeological evidence that they lived side by side for several thousand years," said study team member David Reich.

So how did modern humans with Neanderthal DNA end up in Asia and Melanesia?

Neanderthals, the study team says, probably mixed with early Homo sapiens just after they'd left Africa but before Homo sapiens split into different ethnic groups and scattered around the globe.

The first opportunity for interbreeding probably occurred about 60,000 years ago in Middle Eastern regions adjacent to Africa, where archaeological evidence shows the two species overlapped for a time, the team says.

And it wouldn't have taken much mating to make an impact, according to study co-author Reich. The results could stem from a Neanderthal-modern human one-night stand or from thousands of interspecies assignations, he said.

Genetic anthropologist Jeffrey Long, who calls the Science study "very exciting," co-authored a new, not yet published study that found DNA evidence of interbreeding between early modern humans and an "archaic human" species, though it's not clear which. He presented his team's findings at a meeting of the American Association of Physical Anthropologists in Albuquerque, New Mexico, last month.

Long's team reached its conclusions after searching the genomes of hundreds of modern humans for "signatures of different evolutionary processes in DNA variation."

Like the new Science paper, Long's study speculates that interbreeding occurred just after our species had left Africa, but Long's study didn't include analysis of the Neanderthal genome.

"At the time we started the project, I never imagined I'd ever see an empirical confirmation of it," said Long, referring to the Science team's Neanderthal-DNA evidence, "so I'm pretty happy to see it."